1. Field of the Invention
The present invention relates to sensors and particularly to a differential polarimetric fiber optic sensor.
2. Description of the Prior Art
In one prior art type of polarimetric fiber optic sensor, a compensated-polarimetric fiber optic sensor configuration was used. In such a configuration, a broadband source can be used. In this compensated-polarimetric fiber optic sensor configuration, sensing and reference sections of high birefringence single-mode fiber are spliced directly together with their polarization axes rotated 90.degree. with respect to each other. If the lengths of the two sections are well matched, the effective modal path imbalance experienced by the two polarization components propagating through the overall system is close to zero, and a light source of low coherence length can be used. However, since the sensing and reference sections are coupled directly together, both sections can be subjected to the same measurand, which can introduce errors into the sensor.
A typical difficulty with polarimetric configurations results from the demodulation or "linearization" of the measurand-induced modulation in the polarization mode phase delay in situations where the sensing fiber section is required to operate passively and remotely from the detection electronics. Techniques based on laser frequency modulation, similar to those used for the demodulation of interferometric sensors, have been adopted with non-compensated polarimetric sensors. However, these techniques require sensor fiber lengths of tens of meters to provide a sufficient effective path imbalance between the polarization modes of the fiber for the operation of the technique, and a single frequency diode laser source is necessary to implement the technique.